In case of speech recognition using a word spotting method which recognizes keywords that are included in input voice, by including a garbage model which accepts optional syllable strings as a garbage in descriptive grammar used for the recognition, it could realize the speech recognition relatively easily without describing all sentence patterns and vocabulary words that have possibility to utter as a vocal sound (refer to a non-patent document 1).
Although how it models the garbage influences significantly on accuracy and speed of the recognition, in order to widen a covering scope, in general, it often uses a dumb model and a model which expresses all Japanese syllables (phoneme) as indicated by the patent document 1.    Patent document 1: Japanese Patent Publication No. 3039634    Non-patent document 1: Tomohiro Konuma and Kazuya Takeda, “Consideration of Word Spotting using Garbage Model and Syntactic Restriction”, Acoustical Society of Japan, contributed paper for autumn meeting 2-1-17, pp. 110-111 (1992).